MUSIC: THIS WEEK; A Place in the Pantheon for Strayhorn?

UNTIL recently, the great jazz composer Billy Strayhorn, who died in 1967, endured a strange kind of obscurity. Many knew that he joined Duke Ellington in 1939, that he was partly responsible for the explosion of first-class music to come from the band in the early 1940's and that he collaborated with Ellington on some of his suites in the 1960's. Strayhorn was not invisible, but the quality of his contribution was largely misunderstood.

Then, in 1996, a full Strayhorn biography appeared: ''Lush Life,'' by David Hajdu. And in January, the Dutch musicologist Walter van de Leur got down to chapter and verse in his book ''Something to Live For.'' Mr. Van De Leur studied Strayhorn's handwritten scores, finding more than 100 undiscovered pieces; he argues for Strayhorn's subdued, slightly dissonant, multilayered works to be canonized in their own right.

On Saturday, the fruit of Mr. Van de Leur's decade of research will be performed at St. Peter's Church, at Lexington Avenue and 54th Street in Manhattan at 9 p.m. (Tickets are $25; information is available at 212-675-7477). The program includes both well-known songs (''Chelsea Bridge,'' with a restored chorus that Ellington had excised) and unknown one (the American premiere of ''Cashmere Cutie''). The performance has been commissioned by the New York-based Duke Ellington Society; the band will be led by Michael Hashim, another Strayhornophile. BEN RATLIFF